I like to use the Minimal Facts Approach.

Paul said, "I went to Jerusalem to get to know Peter, and I stayed with him for fifteen days. The only other apostle I met at that time was James, the Lord’s brother" (Gal. 1.18,19). Some years later he wrote, "The leaders of the church had nothing to add to what I was preaching. James, Peter, and John, who were known as pillars of the church, recognized the gift God had given me, and they accepted Barnabas and me as their co-workers. They encouraged us to keep preaching to the Gentiles, while they continued their work with the Jews" (Gal. 2.6,9,10).

In 1 Cor. 15 Paul gave the gospel message he received from them that he likewise had received from Jesus seeing Jesus alive from the dead in various group settings.